1. Field of the Invention
The invention relates to circuit breakers, and more particularly to circuit breakers having shunt trip mechanisms with undervoltage release capability.
2. Description of the Prior Art
Circuit breakers are widely used in industrial, commercial, and residential applications to provide protection for electrical apparatus and distribution equipment. Upon overcurrent conditions through a connected circuit, the circuit breaker will automatically open to interrupt current flow through the circuit. In order to protect multi-phase circuits, the circuit breaker must have a plurality of poles, one for each phase of the circuit being protected. Overcurrent conditions through any one of the several phases must cause the circuit breaker to automatically interrupt the current flow therethrough.
In addition to the previously mentioned overcurrent tripping capabilities, certain applications, such as in the mining industry, require a fail-safe undervoltage release or remote tripping capability. This requires that the circuit breaker be tripped whenever a control voltage falls below a predetermined minimum.
Many multipole circuit breakers in the prior art employed a separate trip mechanism for each pole. These trip mechanisms were connected by a pole-to-pole trip bar or similar mechanism so that actuation of any one of the several trip mechanisms would result in a tripping operation on all poles of the circuit breaker. Such a circuit breaker is described in U.S. Pat. No. 3,422,381, issued Jan. 19, 1969 to Julius Toth and assigned to the assignee of the present invention. Undervoltage release capability could be provided by a hold-in type solenoid connected to an external control line. When voltage on this control line fell below a predetermined value the solenoid would release, striking the pole-to-pole trip bar and releasing the latch mechanism of the circuit breaker.
Requirements for more flexible adjustment of time-current tripping characteristics have resulted in the development of circuit breakers employing a single trip actuator energized by an electronic circuit which senses the current flow through each phase of the multipole circuit breaker. Overcurrent conditions through one or more phases of the circuit breaker will cause the electronic circuit to energize the single shunt trip actuator and cause release of the latch mechanism to separate the circuit breaker contacts. This type of shunt trip actuator eliminates the need for trip bars or other complex pole-to-pole mechanisms. However, it is difficult to modify existing undervoltage release mechanisms for use with circuit breakers which do not employ the pole-to-pole mechanisms. It is desirable, therefore, to provide a circuit breaker incorporating an undervoltage release mechanism compatible with a single shunt trip actuator.
In many circuit breakers, the space available for installation of trip mechanisms and undervoltage release mechanisms is extremely limited. Thus, it is desirable to provide a compact unitary latch release actuator comprising a combined trip actuator and undervoltage release mechanism.
Some circuit breakers using an electronic circuit to energize a single shunt trip actuator employ a flux transfer type of shunt trip actuator wherein a signal from the electronic circuit energizes a coil which overcomes the force provided by a permanent magnet to effect release of the trip mechanism. The circuit breaker described in the aforementioned U.S. patent application Ser. No. 595,184 successfully provides an undervoltage release capability by using a second coil as an electromagnet to replace the permanent magnet of the trip actuator. However, under certain conditions the coils of the two-coil actuator interact in the manner of a transformer, thereby slowing the response of the actuator to a tripping signal. It is desirable to provide a multipole circuit breaker using a single trip actuator having undervoltage release capability and a shorter response time. In addition, it is desirable to provide such a circuit breaker which is lower in cost and of simpler construction.